Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage retaining a reticle, a lens assembly and a wafer stage retaining a semiconductor wafer. The reticle stage and the wafer stage are supported above a ground with an apparatus frame. Typically, one or more motors precisely position the wafer stage and one or more motors precisely position the reticle stage. The images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise relative positioning of the wafer and the reticle is critical to the manufacturing of high density, semiconductor wafers.
A typical reticle stage includes a coarse stage and a fine stage. The coarse stage is used for relatively large movements of the reticle and the fine stage is used for relatively small, precise movements of the reticle. Existing reticle stages typically utilize a pair of spaced apart fine Y motors to move the fine stage along a Y axis and a pair of spaced apart coarse Y motors to move the coarse stage along the Y axis.
Unfortunately, existing reticle stages that utilize both a coarse stage and a fine stage have a relatively large total mass. As a result of the large mass, large motors are needed to move and position the fine stage and the coarse stage. These motors occupy valuable space near the stage, consume large amounts of electric current and generate a significant amount of heat. The heat is subsequently transferred to the surrounding environment, including the air surrounding the motors and the other components positioned near the motors. The heat changes the index of refraction of the surrounding air. This reduces the accuracy of any metrology system used to monitor the positions of the stages and degrades machine positioning accuracy. Additionally, the heat causes expansion of the other components of the device. This further degrades the accuracy of the device.
Moreover, a large mass, reticle stage has a relatively low resonant frequency and a low servo bandwidth. As a result of the low resonant frequency and low servo bandwidth, external forces and/or small reaction forces can easily vibrate and distort the reticle stage. This will influence the position of the reticle stage and the performance of the exposure apparatus.
Additionally, the multiple motors required for both the coarse stage and the fine stage complicates the layout of the reticle stage and the system required to control both the coarse stage and the fine stage.
In light of the above, it is an object of the present invention to provide a stage assembly that has a relatively low mass, a relatively high resonance frequency and a relatively high servo bandwidth. Another object is to provide a stage assembly that is relatively simple to control, allows space for service access, and allows space for a measurement system. Still another object is to provide a stage assembly that utilizes efficient motors to move the components of the stage assembly. Yet another object is to provide a low mass stage assembly that can simultaneously carry two reticles. Another object is to provide a stage assembly that offsets the mass of a fine stage to minimize distortion to a stage base and a lens assembly. Another object is to provide a stage that utilizes reaction force cancellation to minimize the forces transferred to a mounting frame. Still another object is to provide an exposure apparatus capable of manufacturing high density, semiconductor wafers. Yet another object is to provide a stage assembly having a guideless fine stage and a guideless coarse stage.